The present invention relates to an apparatus which may be used for the rapid and even distribution of fibrous material onto a moving surface for the formation of web material.
There are several techniques for the formation of fibrous sheets as webs. Wet-laying is a process by which a liquid suspension containing fibers to be incorporated into a sheet is filtered through a sieve. The fibers in the liquid suspension are laid onto the sieve as the water passes through thereby creating the sheet. This process is most useful for the creation of thin, non-absorbent sheets such as paper. Another well known process is dry-forming. This process involves the suspension of fibers which are to form a fibrous sheet, in air. These fibers are then laid onto a wire screen across which a vacuum is applied to draw the fibers onto the screen and hold them in place. This technology is particularly useful in the production of absorbent products.
There are several dry-forming techniques known in the art. However, a common problem associated with dry-forming fibrous webs is the difficulty in creating an even distribution of the fibers across the web. This problem manifests in the industrial production of dry-formed fibrous webs wherein the fibers are laid continuously on a rapidly moving conveyer belt. The present invention embodies an apparatus to overcome this problem.
The invention relates to a screen pipe used in the dry forming of web material to distribute fiber material blown into the screen pipe through a jacket onto a wire screen arranged to move under the pipe. The fiber material provided inside the screen pipe is made to rotate for example by means of a spiked roll placed inside the jacket, so that the movement of the fiber material has both a radial and a tangential component with respect to the jacket of the screen pipe. The jacket includes, on its inner surface, profiled grooves extended axially along the pipe. The edges of the profiled groove with respect to the tangential component of the fiber flow are positioned at different angels to the tangential component. The bottoms of the profiled grooves include holes or slots through which the fibers are discharged from the screen pipe.
A distribution unit where a screen pipe, as described above, can be used is known for example from Finnish Patent 66,948. This patent describes the basic structure of a distribution unit typically used in dry forming of sheet material. The distribution unit, generally called a former, comprises screen pipes, as described above, placed preferably in pairs to provide fiber flow in opposite directions over the wire in the cross direction thereof. The primary reason for such positioning of the pipes is that, in practice, it is virtually impossible to ensure that fiber flow, which occurs only in one direction over the wire, is deposited evenly onto the wire in the cross direction, thus providing a uniform cross direction web profile. When screen pipes are placed in pairs so that the fiber flows in the pipes in opposite directions, a web""s cross direction profile can be made considerably more uniform. For a web to be of uniform quality, the variation in the web thickness in the cross direction thereof should be minimal. An acceptable deviation from a target thickness is typically xc2x15%.
It has proved to be difficult to provide an even cross direction profile with the distribution unit described in Finnish Patent 66,948, despite the use of screen pipes in pairs. However, the basic structure of the distribution unit is useful and provides a high fiber discharge rate also with rather long fibers.
A screen pipe including, on its inner surface, axial profiled grooves wherein the edge of the groove that is located downstream of the tangential component of the fiber flow and the edge of the groove located upstream of the tangential component are at different angles to the tangential component, is known for example from PCT application WO87/04474. In a screen pipe disclosed in the aforementioned publication, fiber material is caused to move by means of a rotor or a spiked roll placed axially inside the pipe. The rotor or roll rotates in such a direction that the fibers whose movement it activates first come across the edge of the profiled groove which is at a more obtuse angle to the tangent of the jacket than the upstream edge of the groove. This method produces microturbulence, which improves the flow of fibers through holes or slots provided in the screen pipe.
A primary problem with the aforementioned arrangement is the capacity at which fibers can be fed through the screen pipe. The higher the desired wire speed and thus the rate of web formation, the higher the required capacity of the distribution units. The present invention overcomes the disadvantages of uneven fiber distribution and low fiber feed capacity.
The present invention relates to a screen pipe for the expulsion of fibrous material from said screen pipe including a cylindrical screen jacket which may rotate in a single given direction during its normal mode of operation, which includes the following elements on its inner surface:(a) one or more slots which completely penetrate the jacket through which the fibrous material may exit the apparatus; and (b) a groove located immediately downstream of the slot, relative to the direction of rotation of the screen jacket, wherein the edge of the groove which faces the adjacent slot forms an angle with the tangent of the screen jacket between 100xc2x0 and 160xc2x0; and (c)a second groove located immediately upstream of the slot, relative to the direction of rotation of the screen jacket, wherein the edge of the groove which faces the slot forms an angle with the tangent of the screen jacket between 70xc2x0 and 110xc2x0. This screen pipe may further comprise a feed pipe, rotatable brush rollers and a blade wheel. The edges of the grooves may also be substantially straight or curved. The screen pipes of the invention may also be employed in oppositely oriented pairs for dry forming web material. Furthermore, the screen pipe may be operated such that the screen jacket is not rotated.